>>Just looking for hard specification limits.
Do you mean the hard limit for a 64-bit JVM or the hard limit for RHEL?
The 64-bit JVM accepts a long value for max heap size, so look up Long.MAX_VALUE.
According to this page, RHEL running on a 64-bit Intel chip (not Itanium) accepts 1TB or RAM.
http://support.bull.com/ols/product/system/linux/redhat/help/kbf/g/inst/PrKB11417
Of course, this is absolutely useless information (unless you have a motherboard than has enough memory slots for 1TB) - your pratical limit is the amount of RAM you have.